Peelable packaging films are frequently utilized in diverse container sealing applications. Primarily they are used to seal the mouths of foodstuff or medicine containers. In such applications, they firstly provide closure of the mouth to prevent unintended initial loss of the contents. They provide a solid-, vapor- and liquid-tight barrier to prevent contamination and spoilage of the contents. Being non-resealable once opened, they provide a security function by alerting the consumer whether the contents of a presumably unopened container have been potentially tampered with. Films intended for container sealing applications such as these are sometimes referred to as “lidding” film. (See FIG. 1.)
Most basically, conventional lidding films include at least two component layers. First a base layer forms a mechanical closure preventing foreign substances from entering the container. Second a seal layer, which has a melting point or softening point below that of the base layer, which most commonly is thermally adhered to the circumference of the container mouth, provides a seal removable by peeling, preferably without tearing or leaving sealant material on the container. Many sophisticated modifications and additions to these basic components have developed to provide advantageous features to peelably lidded containers.
A particular disadvantage of some peelable-lidded containers is that the adhesion of the lidding film to the container is so strong that the film is difficult to remove from the container without destruction of the seal layer or tears in the entire film cross-sections; in such cases, the strong adhesion can undesirably wholly or partially delaminate the lidding film leaving pieces adhered to the lip of the container mouth. If the adhesion to the container mouth is made too weak, the lidding film can be removed inadvertently or prematurely by many incidental causes including, for example, pressure and movement of the contained material. Ideally a seal layer that combines a good seal strength (typically in excess of 0.7 lb/in) and also a clean peel, free of tears is desired. A further requirement is that a seal with good peel characteristics can be obtained over a wide temperature range, typically between 350 and 420° F. for CPET (crystalline polyethylene terephthalate) and between 200-300° F. for APET (amorphous polyethylene terephthalate) trays. Within each range, it is desirable that the seal strength profile vs. temperature be as flat as possible, to allow interchangeability between different packaging sites using different sealer settings.
Many sealable films known in the art are coextruded. Films made by coextruding PET as a base layer with an amorphous PET copolymer (typically obtained by substituting, e.g. 18 mole % of the terephthalic acid with isophthalic acid or by substituting up to 33 mole % of the ethylene glycol with cyclohexanedimenthanol) are often used in applications requiring permanent sealing of the film to itself, e.g. bag construction. While these coextruded films are cost-effective to produce, the films obtained are not truly peelable as they often tear and split when removed from the package, often sticking to the food, especially to foods that contain sauces and cheese such as pizza. One reason for this is that seal materials that are coextrudable with PET (such as the copolyester of ethylene glycol with about 82 molar parts terephthalic acid/and 18 molar parts isophthalic acid or the copolyester of terephthalic acid with about 33 mole % of 1,4-cyclohexane dimethanol with about 67 mole % of ethylene glycol (for example, Eastman Chemical Co.'s Eaststar™ grade PETG™ 6763) require a high melt viscosity in order to be compatible with PET in coextrusion. As a result, of this high viscosity, they cannot form adequate seals on CPET trays unless the seal temperature approaches that of the melting point of the CPET substrate. This, combined with the fact that such materials have typically glass transition temperature (Tg) above room temperature results in destructive seals.
Several patents propose as a remedy coextrusion of a seal layer consisting of a copolyester partially based on aliphatic dicarboxylic acids (such as adipic, sebacic, azelaic acids) and partially based on aromatic acids such as terephthalic acid, that have glass transition temperatures below room temperature (typically in the range between −0° C. and 10° C. and melt temperatures generally below 160° C.). A melting temperature (Tm) in this range enables sealing at temperatures suitable for CPET or APET trays whereas the Tg below room temperature enables peeling characterized by plastic deformation (yield) which enables the film to sustain stresses resulting from peeling forces without suffering brittle failure. U.S. Pat. Nos. 7,205,040; 7,141,293; 7,329,453; and 7,442,427 describe coextruded seal layers comprising copolyesters based on aromatic/aliphatic diacarbocylic acid combinations providing peelable seal strengths in the range of 1.1 to 3.4 lb/in at a seal temperature of 392° F., depending on exact copolyester composition and skin thickness. Although those patents do not allude to any issues related to coextrusion stability, it has been the experience of the applicant that a disadvantage of coextrusion processes consisting of skin layers of copolyesters based partially on aliphatic diacids is the frequent occurrence of flow instabilities related to viscosity mismatch between the less viscous partially aliphatic diacid-based copolyester skin and the higher-viscosity polyethylene terephthalate core; even if a lower melt extrusion temperature is employed on the skin or satellite extruder, the die block has to be kept at a temperature above the melting temperature of the PET homopolymer, (typically above 505° F.) which reheats the skin material to a temperature where its viscosity becomes too low resulting in gross irregularities.
Because of the difficulty in consistently coextruding with PET a suitable hot melt adhesive skin layer material providing peelable seals, alternative methods such as offline or inline coating techniques have been employed.
Off-line coating can be conducted either by melt extrusion (extrusion coating) or by solvent coating, which bears the additional cost and environmental risk of handling organic solvents.
Exemplary peelable, sealable films used for lidding and packaging foods known in the art are described in U.S. Pat. No. 5,888,599. These films are prepared by a multi-step process in which a core film is first prepared comprising a base layer which is then solvent-coated with a hot melt adhesive in a separate step (off-line coating;) to form a skin layer whose function is to provide a peelable heat seal. Peelable seal strengths up to 0.88 lb/in have been attained at seal temperature 180° C. and seal layer thickness of about 2 μm; however solvent-coating techniques present the disadvantage of additional costs and environmenal risks associated with handling and recycling organic solvents
Sealable extrusion coatings laminated off-line can be either polyesters (such as those described for example in US patent application US2004/0052993 or vinyl polyolefins such as ethylene vinyl acetate (EVA). A disadvantage of off-line coated polyester sealants is that, if done by extrusion coating, that process results in relatively thick coatings (typically 20 μm or higher) which increase the overall cost significantly; or, if applied by solvent coating, the solvents have the disadvantages listed above. EVA coatings have the disadvantage of poor adhesion to the PET base film; for that reason, additional primer and tie layers need to be applied during extrusion coating, which increase overall cost and complexity.
Inline coating methods are preferable because they result in lower thickness; in such methods, the coating is conducted between the machine direction orientation and the transverse direction orientation steps that are parts of a typical biaxial orientation film production process. For instance, U.S. Pat. No. 6,939,584 describes an inline extrusion coating method for obtaining a heat-sealable and peelable skin layer consisting of a copolyester derived from one or more dicarboxylic acids and one ore more glycols. In the examples, a coating comprised of a copolyester of sebacic acid/terephthalic acid/butylene glycol (50/50/100) having a Tg of −40° C. and a Tm of 117° C., with final thickness 1.5-2 μm, provided a heat seal strength of 1.87 lb/in on an APET/CPET tray (sealing on the APET surface) at a sealing temperature of 180° C. (356° F.). Identical seal strength was provided by copolyester of azelaic acid/terephthalic acid/ethylene glycol (45/55/100) having a Tg of −15° C. and a Tm of 150° C. Whereas such a sealing temperature is suitable for an APET/CPET tray (thanks to the CPET support) it is too high for a 100% (transparent) APET tray due to excessive distortion. Typical APET trays are sealed at temperatures of 300° F. or lower. At such a temperature, the aforementioned compositions are expected to provide lower seal strengths.
The above patent also recognises the benefit of the base film itself comprising two distinct layers, where the layer upon which the coating composition is applied may comprise a polymer which itself has heat-sealability. The explanation provided is that the heat-sealable layer presence in the base film helps provide a stronger bond of the coated layer on to the tray or that it results in material savings in terms of allowing a lower thickness of the coating layer. The applicant has found that an additional benefit from the presence of a heat-sealable layer in the base film is that it facilitates good adhesion of the coating layer to the base film.
Alternatively to inline extrusion coating, inline coating with aqueous suspensions of hot-melt polymeric substances is also possible, provided that formation of stable suspensions and achievement of coalescence during the transverse orientation step are possible. For example, WO A-96/19333 describes a process for producing peelable films, in which the heat-sealable, peelable layer is applied inline to the polyester film from an aqueous dispersion that also contains about 10 wt % of an organic solvent. In the example provided, the heat-sealable material is a copolyester of terephthalic acid/adipic acid/sulfoisophthalic acid/butane-diol, ethylene glycol, and polybutane-diol. A peel strength of 0.82 lb/in is achieved with a coating thickness of 1.5 μm at a sealing temperature of 180° C. (356° F.) on an APET/CPET tray. Again, this temperature would be too high for a 100% APET tray and the expected seal strength is expected to be lower at the maximum seal temperature used of APET trays (around 300° F.).
Despite the extensive prior art in the preparation of laminates consisting of a PET base layer and a copolyester made from a combination of aliphatic and aromatic dicarboxylic acids as the seal layer by coextrusion, offline or inline coating methods, previous investigators failed to recognize the importance of controlling the crystallinity developed in the seal layer during the cooling process of the coating towards attaining exceptionally high but also peelable seal strengths at temperatures lower than those exemplified in the prior art, which make strong seals to APET possible.
The applicant has found that by proper selection of the aromatic/aliphatic copolyester composition the goal of achieving high peelable seal strengths, at relatively low temperatures, by extrusion coating methods, can be achieved.